This invention relates to an automatic transmission for an automative vehicle having a feedback control function for supplying an adequate hydraulic pressure to frictional engaging elements of the automatic transmission during a shift from one speed ratio to another. More particularly, the present invention relates to a method of automatically correcting the initial hydraulic pressure to set it at a suitable value to be supplied to the frictional engaging elements of the automatic transmission even when the engine of the vehicle is replaced by another of different capacity such as an output torque or displacement different from that of the former.
In general, an automatic transmission for an automative vehicle includes frictional engaging elements such as clutches and brakes which are selectively actuated by the application of hydraulic pressure to stop rotation of a selected one of a plurality of rotary elements relative to another, or connect or disconnect the rotary element from an input shaft, thereby automatically changing over or shifting the speed ratio depending on the operating condition of the vehicle. In order to protect the automatic transmission against damage and to ensure a comfortable ride for occupants of the vehicle, the hydraulic pressure supplied to the frictional engaging elements of the automatic transmission is controlled gradually along a predetermined characteristics curve from the value of the initial hydraulic pressure supplied immediately after a shift, to a value corresponding with another speed ratio.
Nowaday, a variety of kinds of vehicles are sold on the market to meet a variety of needs, and a variety of kinds of engines are furnished to be mounted on such vehicles. However, due to the fact that the initial hydraulic pressure described above differs depending on the factor such as the displacement and/or output torque of engines mounted on vehicles, a variety of kinds of automatic transmissions must also be prepared to meet the variety of kinds of the engines. Suppose, for example, the case in which an automatic transmission adapted to be combined with an engine having a relatively large displacement is combined with an engine having a relatively small displacement. In such a case, since the operating hydraulic pressure (line pressure) of the automatic transmission adapted to be combined with a large-displacement engine is primarily high compared with that of the automatic transmission adapted to be combined with the small-displacement engine, the engaging force of the frictional engaging elements will become excessively large compared with the output torque of the engine, and the frictional engaging elements will be engaged by the initial hydraulic pressure as soon as a shift starting signal is generated, resulting in impactation of a great shift shock. In a converse case, on the other hand, the line pressure will be too low to start a speed shift or a large length of time will be required to attain the speed shift. Thus, it has been impossible to apply an automatic transmission of single capacity to an engine other than those of predetermined capacity.
Therefore, automatic transmission of a small number had to be manufactured for each of a variety of kinds, resulting in increased production costs and troublesome production control.